1. Field of the Invention
The present invention relates to a microphone unit which is formed in a semiconductor chip and comprises a pressure sensitive element such as an electret capacitor, and also to a microphone filter for removing direct-current (DC) components and low-frequency components unnecessary for a sound signal from an output signal from the microphone unit.
2. Description of the Background Art
A conventional microphone unit and microphone filter are shown in FIG. 4. FIG. 4 illustrates a microphone unit MU2 comprising an electret capacitor EC. Upon receipt of sound pressure, the electret capacitor EC varies its capacitance and generates an input signal Vin between its both electrodes. A ground potential GND is applied to one end of the electret capacitor EC. Then, an impedance converter comprised of diodes D1, D2, a resistance R1, and N-channel MOS transistors T1, T2 is connected across the electret capacitor EC. More specifically, the anode of the diode D1 is connected to one end of the electret capacitor EC and the cathode thereof to the other end of the electret capacitor EC. The diode D2 is connected across the electret capacitor EC with its anode and cathode connected in the reverse fashion for those of the diode D1. The resistance R1 is connected in parallel across the electret capacitor EC. The source of the transistor T1 is connected to one end of the electret capacitor EC and the gate thereof to the other end of the electret capacitor EC. The drain of the transistor T1 is connected to the source of the transistor T2. A power supply potential Vdd is applied to the drain of the transistor T2 and a predetermined potential Vref2 to the gate of the transistor T2. Further, the ground potential GND is applied to the back gates of the transistors T1 and T2.
When no input signal Vin is applied, the gate-source voltage of the transistor T1 is maintained at 0 V by the diodes D1, D2 and the resistance R1. Upon application of the input signal Vin, variations occur in the gate-source voltage of the transistor T1. This effects a change in the drain-source current. In the transistor T1 being of a depletion type, the current flows between the drain and source even if the gate-source voltage is 0 V. The variations in the drain-source current of the transistor T1 causes variations in the drain-source current of the transistor T2, thereby changing the gate-source voltage of the transistor T2. This potential change at the source of the transistor T2 becomes an output signal Vout2.
As shown in FIG. 4, a microphone filter FT2 is configured as a CR circuit composed of a capacitor C1 and a resistance R4. The capacitor C1 receives at its one end the output signal Vout2 from the microphone unit MU2 and is connected at its other end to one end of the resistance R4. Further, a predetermined potential Vref1 is applied to the other end of the resistance R4.
The microphone filter FT2 removes DC components and low-frequency components included in the output signal Vout2 by outputting a voltage dropped at the resistance R4. Since serving as a sound signal, the output signal Vout2 should have an audio-frequency region in the range of approximately 100 Hz to 20 kHz. Thus, DC and low-frequency components unnecessary for the sound signal are removed from the output signal Vout2.
The output from the microphone filter FT2 is fed into an amplifier. Illustrated in FIG. 4 is an amplifier including a voltage follower and an inverting amplifier. More specifically, the output from the microphone filter FT2 is fed into a positive input of an operational amplifier OP1. The operational amplifier OP1 receives its own output at its negative input, serving as a voltage follower. The output from the operational amplifier OP1 is then fed into a negative input of an operational amplifier OP2 through a resistance R2. Also, the operational amplifier OP2 receives its own output Vout3 at its negative input through a resistance R3, serving as an inverting amplifier. Here, a predetermined potential Vref1 is applied to the positive input of the operational amplifier OP2.
The microphone filter FT2 removes DC and low-frequency components from the output signal Vout2 at a cut-off frequency, f=1/(2xcfx80CR), where C is the capacitance of the capacitor C1 and R is the resistance of the resistance R4. In order to remove low-frequency signals approximately below 100 Hz and DC components from the output signal Vout2, the product of the capacitance C and the resistance R, i.e., time constant, must be large; for example, such a combination as the capacitance of 1 xcexcF and the resistance of 1.6 kxcexa9 or the capacitance of 100 pF and the resistance of 16 Mxcexa9 becomes necessary. Forming such high capacitance and resistance in combination in a single semiconductor chip increases chip area, preventing downsizing and cost reduction of semiconductor chips. For this reason, the conventional microphone filter FT2 cannot fit in a semiconductor chip in which the microphone unit MU2 is formed, and other discrete parts such as a capacitor and a resistance are required to form the filter.
Even with the use of discrete parts such as a capacitor and a resistance, it is difficult to achieve downsizing and cost reduction because of a high cost of such parts, an increase in processing steps, and the impossibility of housing the microphone filter in the semiconductor chip in which the microphone unit is formed. Incidentally, the amplifier is not fitted into the same semiconductor chip as the microphone unit is formed.
A first aspect of the present invention is directed to a microphone filter comprising: a capacitor having one end, and the other end to which an output from a microphone is fed; a first transistor having a first current electrode which is connected to the one end of the capacitor, a second current electrode to which a first fixed potential is applied, and a control electrode; a second transistor having a first current electrode, a second current electrode which is connected to the second current electrode of the first transistor, and a control electrode which is connected to the control electrode of the first transistor; and a constant current source connected to the first current electrode and the control electrode of the second transistor.
A second aspect of the present invention is directed to a microphone unit comprising: a microphone formed in a semiconductor chip; and a microphone filter of the first aspect, which is formed in the semiconductor chip, wherein an output from the microphone is fed into the other end of the capacitor.
According to a third aspect of the present invention, the microphone unit of the second aspect further comprises: an amplifier formed in the semiconductor chip and having an input which is connected to the first current electrode of the first transistor of the microphone filter.
A fourth aspect of the present invention is directed to a microphone unit comprising: a microphone formed in a semiconductor chip; and an amplifier formed in the semiconductor chip and having an input into which an output from the microphone is fed.
The microphone filter of the first aspect can utilize, as its resistance, a differential resistance which is produced by a channel length modulation effect or an Early effect of the voltage-current characteristics between the first and second current electrodes of the first transistor. This achieves a large-time-constant microphone filter containing resistance and capacitance as its constituents. Since the first and second transistors and the constant current source form a current mirror circuit, the microphone filter is resistant to variations in the voltage-current characteristics of the first transistor due to temperature changes, and can be formed in a semiconductor chip without a significant increase in chip area.
In accordance with the second aspect, the microphone filter of the first aspect and the microphone can be formed in the same semiconductor chip. This achieves downsizing and cost reduction of the microphone unit.
The microphone unit of the third aspect further comprises the amplifier. This achieves further downsizing and cost reduction of the microphone unit.
In accordance with the fourth aspect, the amplifier and the microphone can be formed in the same semiconductor chip. This achieves downsizing and cost reduction of the microphone unit.
An object of the present invention is to achieve a large-time-constant microphone filter which contains resistance and capacitance as its constituents and can be formed in the same semiconductor chip as the microphone unit, thereby ensuring downsizing and cost reduction of the microphone unit.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.